Recently, interest in microreactors has grown rapidly. The term “microreactor” generally refers to an apparatus which includes fine microchannels having a size of about 1 μm to about 1 mm inside the apparatus and which performs reaction in the microchannels. Microreactors have potential to innovate the chemical industry.
Characteristic features of the aforementioned microreactors in relation to organic synthesis include the following: (1) synthesis can be performed from microamounts of source materials; (2) a large surface area is provided per unit volume (flow); (3) temperature control is remarkably easy; (4) interface reaction can be caused to occur at high efficiency; (5) reaction time, cost, and environmental load can be reduced; (6) reaction can be performed in a sealed system, enabling safe synthesis of toxic and hazardous substances; (7) contamination is prevented by virtue of a small-scale closed system; and (8) mixing, product isolation, and purification can be effectively performed through employment of laminar flow through microchannels.
From the viewpoint of industrial applications, (a) production amount can be potentially elevated through increasing the number (numbering-up) of microchannels while maintaining the dimensions of microchannels unchanged. That is, a step for producing a test intermediate apparatus, which has been conventionally required upon transfer of a laboratory apparatus to a plant, can be eliminated. Therefore, employment of microreactors has the following advantages: (b) a new production process can be immediately inaugurated at low cost; (c) experimental results achieved in the laboratory can be immediately transferred to a plant; and (d) industrial production can be conducted in a small-scale plant.
Some cases in which chemical reaction is performed by means of such a microreactor have been disclosed. For example, disclosed are a method for carrying out chemical reaction (see, for example, Japanese Kohyo Patent Publication No. 2001-521816); production of aldols through employment of a microstructure reaction system (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2002-155007); nitrification in a static micromixer (see, for example, Japanese Kohyo Patent Publication No. 2003-506340); and a method for producing arylboron compounds and alkylboron compounds by means of a microreactor (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2003-128677).
Use of a microreactor in polymerization reaction is also disclosed (see, for example, Anal. Chem., Vol. 74. p. 3112 (2002)). Specifically, polymerization of ethylene is carried out in the presence of a metallocene catalyst under pressurized and laminar flow conditions in a flow path (diameter: 1.27 mm). The above polymerization, which is coordination polymerization employing a metallocene catalyst, is a technique completely different from the radical polymerization of the present invention. A method for producing a radical polymer by use of a micromixer is also disclosed (see, for example, Japanese Kohyo Patent Publication No. 2002-512272). In the method, a radical-polymerizable monomer and a polymerization initiator are mixed by use of a micromixer where mixing is performed in narrow flow paths, followed by polymerization. As a result, formation of a high-molecular weight in the produced polymer is suppressed, and precipitation in the reactor is avoided. A characteristic feature of this technique lies in carrying out mixing of a monomer and an initiator in a microspace, and polymerization per se is performed in a tube reactor having a diameter of some centimeters.
Radical polymerization is an essential technique which enables polymerization of a large number of monomer molecules. Therefore, radical polymerization is widely employed in industry as means for producing a variety of polymers. During radical polymerization, a large amount of reaction heat is generated. Thus, when either the batch method or the continuous method is employed, polymerization is generally performed under mild reaction conditions over a long period of time in order to remove reaction heat, making production efficiency problematically poor. In a conventional polymerization method, reaction heat readily causes unevenness in polymerization temperature at a reaction site. Furthermore, when the polymerization is performed in a continuous manner, the reaction mixture does not readily form laminar flow, resulting in variation in residence time in the reaction site. Thus, the formed polymer readily assumes a mixture of polymers having a variety of molecular weights, which is problematic.
Meanwhile, in fabrication of microreactors, microflow paths are generally produced through a highly elaborate processing technique such as photolithography, etching, or fine mechanical processing. Therefore, difficulty has been encountered in employing microreactors for carrying out chemical reaction.